Method of cutting tissue using a laser

ABSTRACT

A method of cutting tissue for use as an implantable medical device employs a laser cutting system. The laser cutting system is computer controlled and includes a laser combined with a motion system. The laser precisely cuts segments out of source tissue according to predetermined pattern as designated by the computer. The cutting energy of the laser is selected so that the cut edges of the tissue segments are fused, melted or welded to discourage delamination or fraying, but communication of thermal energy into the segment beyond the edge is minimized to avoid damaging the tissue adjacent the edge.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/007,732, filed Dec. 8, 2004 (now U.S. Pat. No. 7,594,974), which is acontinuation of U.S. application Ser. No. 10/207,438, filed Jul. 26,2002 (now U.S. Pat. No. 6,872,226), which claims the benefit of U.S.Provisional Application No. 60/308,268, filed Jul. 26, 2001, and whichis a continuation-in-part of U.S. application Ser. No. 09/772,526, filedJan. 29, 2001 (now U.S. Pat. No. 6,682,559), which claims the benefit ofpriority to U.S. Provisional Application No. 60/178,333 filed Jan. 27,2000. All of the foregoing are incorporated by reference in theirentireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to implantable medical devices, and moreparticularly relates to forming segments used to construct suchimplantable medical devices.

2. Description of the Related Art

Medical devices are often surgically implanted into a patient in orderto assist or replace diseased tissue. For instance, a prosthetic devicesuch as an artificial heart valve can be implanted to replace adefective natural heart valve.

It is important for such prosthetic devices to be substantially durable,as failure of the device may have drastic consequences for the patient.As can be appreciated, a prosthetic device that wears out prematurelymay put a patient at substantial risk, both because of the possibilityof early, sudden failure of the device and because of additional surgerythat may be required to replace the device.

Some implantable medical devices comprise two or more members orsegments of material that are assembled to form the device. The mannerin which the segments of material are formed can significantly affectthe durability of the device. For example, if the segments are formed bybeing cut out of a larger portion of material, the edges of the cutsegments may be especially susceptible to premature wear. Also,imprecise cutting or inconsistencies between cut segments may negativelyaffect both the operability and durability of the assembled prostheticdevice.

SUMMARY OF THE INVENTION

Accordingly, there is a need for a method and apparatus for cuttingsegments of material for use in implantable medical devices wherein thesegments are cut with precision and consistency, and wherein the cutedges of the segments resist wear when implanted into the body.

In accordance with one embodiment, a method of creating an implantablemedical prosthesis is provided. A sheet of pericardium having at leasttwo tissue layers is provided and a segment of tissue is cut out of thesheet of pericardium with a laser beam. The cutting comprises operatinga laser at a power and pulse rate such that the beam welds the layers ofthe pericardium together along a laser cut edge without significantlyburning the pericardium adjacent the cut edge.

For purposes of summarizing the invention and the advantages achievedover the prior art, certain aspects and advantages of the invention havebeen described hereinabove. Of course, it is to be understood that notnecessarily all such aspects or advantages may be achieved in accordancewith any particular embodiment of the invention. Thus, for example,those skilled in the art will recognize that the invention may beembodied or carried out in a manner that employs one or more aspects toachieve or optimize one advantage or group of advantages as taughtherein without necessarily using other aspects or achieving otheradvantages as may be taught or suggested herein.

All of these aspects are intended to be within the scope of theinvention herein disclosed. These and other aspects of the presentinvention will become readily apparent to those skilled in the art fromthe following detailed description of the preferred embodiments havingreference to the attached figures, the invention not being limited toany particular preferred embodiment(s) disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a prosthetic aortic heart valveconstructed by joining three independently formed leaflets together.

FIG. 2 shows a flat pattern for a leaflet to be used in constructing theheart valve of FIG. 1.

FIG. 3 shows two adjacent leaflets of the valve of FIG. 1 during valveassembly.

FIG. 4 is a top view showing the leaflets of FIG. 3 folded over eachother in a desired manner to form a commissural tab.

FIG. 5 is a scanning electron microscope image of an edge of an equinepericardium segment that has been cut with a razor.

FIG. 6 is a schematic view of a plotted laser cutting apparatus forprecision cutting of segments for implantable medical devices.

FIG. 7 is a plan view showing several aortic valve leaflets arranged tobe cut by the plotted laser apparatus of FIG. 6.

FIG. 8 is a scanning electron microscope image of an edge of a segmentof equine pericardium that has been cut with the plotted laser cuttingapparatus of FIG. 6.

FIG. 9 is a schematic view of another embodiment of a plotted lasercutting apparatus for precision cutting of segments for implantablemedical devices.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention can be used to cut out segments used whenconstructing several types of prostheses. One type of prosthesis thatparticularly benefits from use of the present invention is a replacementheart valve having one or more leaflets that are cut from a sourcematerial and assembled to form the valve. FIGS. 1-4 present a prostheticaortic heart valve 20 constructed in accordance with an embodiment ofthe present invention. This heart valve 20 is discussed in order to helpillustrate aspects and advantages of the invention and is discussed inmore detail in the above-referenced application entitled PROSTHETICHEART VALVE. It is to be understood that other types of implantableprostheses may also benefit from the aspects discussed below.

The aortic heart valve 20 of FIGS. 1-4 comprises three leaflets 22 thatare cut out of a generally flat, flexible source material. Each of thethree leaflets 22 is cut out according to the pattern shown in FIG. 2.As shown, each leaflet 22 has a main body 24 that is scalloped at bothits proximal and distal ends 26, 28. First and second distal tabportions 30, 32 extend outwardly from corresponding first and secondside edges 34, 36 of each leaflet's main body 24. The tabs 30, 32 aresubstantially rectangular in shape and extend distally beyond the distalend 28 of the main body 24.

Each of the tabs 30, 32 communicate with the leaflet main body 24through a neck portion 40. Curved transition edges 42, 44 connect aninner edge 46 of each tab 30, 32 with the distal end 28 of the leaflet22, and a proximal edge 48 of each tab 30, 32 with the correspondingside edge 34, 36 of the leaflet 22. An elongate slot 50 is formed in thesecond tab 32. The slot 50 extends distally from the proximal edge 48 ofthe tab to a point just distal of the distal-most edge 28 of the leafletmain body 24.

With reference next to FIG. 3, adjacent leaflets are connected byaligning the first outer edge 34 of one leaflet with the second outeredge 36 of the adjacent leaflet so that the inner faces of the leafletsengage one another. The side edges 34, 36 are sutured together using aseries of locked stitches 52 arranged along a fold line L_(F) adjacenteach side edge 34, 36.

The series of sutures 52 terminates prior to reaching the proximal edge48 of the tabs 30, 32, with the last suture being placed proximal of theproximal transition edge 44. The tabs 30, 32 are then folded backwardlyalong the fold line L_(F) so as to overlap the outer surface of theirrespective leaflets 22, as shown in FIG. 3. With reference next to FIG.4, the adjacent first and second tabs 30, 32 are folded over one anotherin order to form commissural tabs 56. More specifically, the second tab32 is folded so that the slot 50 straddles the neck portions 40 of bothtabs 30, 32. The first tab 30 is folded opposite the second tab 32 andgenerally aligned with the second tab 32, as shown in FIG. 4. The foldedtabs 30, 32 are then sewn together in order to form the commissural tabs56 shown in FIG. 1.

In the illustrated embodiment, each of the leaflets 22 is substantiallyidentical in shape. It is to be understood, however, that otherprosthetic devices may employ segments of varying sizes and shapes. Forexample, a prosthetic mitral heart valve can employ two leaflets whichare shaped differently from one another. However to maintain consistencyin manufacture, the respective leaflets preferably are substantiallyidentical in size and shape from valve to valve. Additionally,prosthetic devices such as surgical patches may desirably be produced inseveral sizes and shapes.

Replacement valves such as the aortic valve 20 illustrated in FIGS. 1-4are used to replace diseased natural valves. The natural valve is cutout of its place and removed, leaving a valve annulus and a plurality ofdownstream attachment locations. The inflow annulus of the replacementvalve is configured to fit into the valve annulus vacated by the nativeaortic valve. The commissural attachment tabs 56 can be attached to theaorta at points vacated by the native valve's commissural attachmentlocations. The replacement valve thus totally replaces the native valve.

Once installed, the replacement valve functions much the same as anative aortic valve. During systole, the leaflets 22 are forced apart sothat blood flows freely through the valve 20 and into the aorta. Duringdiastole, the leaflets are drawn toward each other and approximate eachother, thus sealing the valve. The commissural attachment tabs 56 helpprevent the valve leaflets from prolapsing during diastole.

In the illustrated embodiment, the leaflets can be constructed ofbiological or synthetic materials. For example, explanted human oranimal tissue, such as bovine, porcine and kangaroo pericardium tissuemay be appropriately used. Synthetic material, such as polyesters,Teflon®, fluoropolymers, woven or knitted cloth, etc. can also be used.Of course, biological and synthetic materials not listed above can beused if appropriate. Leaflet materials for the illustrated heart valvecan be selected using a general guideline that the more pliable, thinand strong a material is, the better. Additionally, it is advantageousfor the material to be as nonthrombogenic as possible.

In a preferred embodiment, the flexible material comprises equinepericardium that has been crosslinked and fixed in a low-concentration,buffered glutaraldehyde solution. Leaflets formed from this material arepliable and easy to open and close.

Equine pericardium that has been treated as discussed above can besupplied as a generally flat, thin and flexible sheet of material fromwhich a plurality of leaflets can be cut. Other source materials, suchas bovine pericardium and woven cloth, can also be obtained in flatsheets. Still further source materials may be obtained in irregular orcurved shapes. For example, segments of intestinal tissue, some knittedcloths and some extruded polymers can be supplied having generallytubular geometry. Segments can be cut from such suitable sourcematerials and then assembled to form the desired prosthesis. Variouscutting media and methods, such as a razor, die cutter, laser or jet offluid and/or particles can be used to cut segments from source material.In a preferred embodiment of the aortic heart valve discussed above,individual valve leaflets are cut from a sheet of treated equinepericardium.

With next reference to FIG. 5, equine pericardium has a laminarstructure with three fibrous layers, the visceral 60, serosa 62, andparietal layers 64. Applicant has discovered that cutting equinepericardium using a contact-type cutter such as a razor or cutting diehas a tendency to delaminate one or more of the layers along the cutedges. FIG. 5 is a scanning electron microscope image of an equinepericardium segment edge 66 that has been cut with a razor.

As can be seen in FIG. 5, each of the layers 60, 62, 64 has a generallydifferent consistency. Additionally, the fibrous material 68 within eachlayer has several discontinuities and gaps 70. In this configuration,the cut edge 66 is especially susceptible to degradation due to externalfactors. For example, a fluid such as blood can fill some of the gaps 69between the layers 60, 62, 64 or fibers 68 and can act as a wedgegradually disconnecting the layers or fibers from one another. Over timesuch delaminations would advance beyond just the cut edge, and maycompromise the performance and strength of the prosthetic segment.

Delaminations of the fibrous layers of a heart valve leaflet can disruptvalve operation and significantly impair valve durability. For example,blood that enters between delaminated layers can cause a cuspal hemotomaor lead to calcification of the valve due to increased turbulence.Additionally, the strength of the leaflet can be reduced. Accordingly,it is desirable to reduce or eliminate delamination of the pericardiumlayers when constructing valves.

Other flexible materials used for heart valves, especially pericardialtissues, may have similar laminar structure, and may be subject tosimilar issues with regard to delamination. Challenges also arise whencutting synthetic materials such as woven or knit polymers, because thecut filaments or yams may have a tendency to fray. Such fraying cancause problems similar to delamination.

In accordance with one embodiment, a laser cutting apparatus 70 isprovided for cutting prosthetic segments from source material 90. Withreference specifically to FIG. 6, the laser cutting apparatus 70comprises a laser system 72 and a computer 74. The laser system 72comprises a laser tube assembly 76, a motion system 78 and a supportplatform 80. The laser tube assembly 76 is configured to create a laserbeam 82 which is directed through a series of optic elements such asmirrors 84 and lenses 86 in order to direct a focused laser beam 88 onthe support platform 80, which is configured to support the sourcematerial 90. The focused laser beam 88 is configured to cut through thesource material 90 in order to cut out a segment according to aprescribed pattern.

The motion system 78 preferably is arranged to selectively locate andmove the position of the focused laser beam 88 relative to the platform80 in order to cut the segment out of the source material 90. In theillustrated embodiment, the motion system 78 can move the laser beam'sposition along horizontal X and Y axes. The support platform 80 isvertically movable along a vertical Z axis. It is to be understood that,in other embodiments, other types of motion systems cap be employed.

The computer 74 preferably controls the laser system 72 via a printerdriver 92, which communicates data from the computer 74 to the lasersystem 72 in order to control laser parameters and motion. In theillustrated embodiment, a computer assisted design (CAD) softwareprogram, such as Corel Draw®, is hosted by the computer 74. The CADsoftware is used to create designs of segments that will be cut. FIG. 7shows a cutting pattern or template 96 created by CAD software. Thetemplate 96 functions as a target for the laser. The illustratedtemplate 96 is configured so that four valve leaflets will be cut from asheet 98 of source material.

In a preferred embodiment, the CAD software also functions as a commandinterface for submitting cutting patterns 96 to the laser system 72through the printer driver 92. When directed to do so by the computer 74and printer driver 92, the laser system 72 precisely cuts the patterns96 from the source material 90.

The laser cutting apparatus 70 is configured to have a pulse power,cutting speed, and number of pulses per inch that will impart sufficientenergy to vaporize portions of the source material along a cut line inorder to cut the desired segment shape, and to at least partially meltthe cut edges. Melting the cut edges effectively fuses or welds thelayers and fibrous matter together.

Welding of the edges is especially advantageous for laminar materialssuch as pericardium, because the melted edge resists delamination. FIG.8 is a scanning electron microscope image taken along a cut edge 100 ofa sample of equine pericardial tissue that has been cut using a laser.When compared with FIG. 5, FIG. 8 shows that the characteristics of thelaser-cut edge 100 are much different than the razor-cut edge 66. Asshown in FIG. 8, the visceral, serosa and parietal layers are no longerdistinguishable when the material has been laser cut. Additionally, thegenerally fibrous, layered character of the pericardium has been changedalong the cut edge 100. Applicant has found that heart valve leafletswith melted edges exhibit dramatically increased durability overleaflets that have been cut using more traditional die-cutting orrazor-cutting methods.

An issue that arises during laser cutting is management of thermalenergy. Excessive thermal energy absorbed by a source material such aspericardium can burn the material. Burning of the material can result inseveral types of damage. For example, the burned material can becomestiff and brittle or can become biased to bend in a particulardirection. Further characteristics of burning include discoloration oreven charring of the material.

Burned portions of a segment of material can jeopardize the integrityand durability of the entire segment, and of a prosthesis constructedusing that segment. For example, a stiffened or biased portion of aprosthetic heart valve leaflet will not move in the same manner as therest of the leaflet during opening and closure of the valve. Thehemodynamic performance of the valve thus could be compromised. Further,damage caused by burning of the material generally weakens the materialand could reduce the durability of the valve. As such, it is desirableto weld the material at the cut edge, but avoid communicating thermalenergy into the cut segment beyond the edge.

Excessive burning of the laser cut edge can also have a negative impact.If excessive laser energy is applied to the cut edge, it is more likelythat thermal energy will be conducted beyond the edge and into thesegment, resulting in tissue necrosis. Additionally, the tissue at anexcessively burned edge may have a somewhat inconsistent thickness,having portions that are significantly thicker than other portions ordeveloping beads of melted material. Discoloration of the cut edge canindicate application of excessive thermal energy. Inconsistencies in theedge make the segment more difficult to work with during manufacture andcan affect performance of the segment. As such, it is desirable to weldthe material at the cut edge in a manner so that the melted edge isrelatively uniform in thickness and consistency and exhibits minimal, ifany, beading.

In a preferred embodiment, a CO₂ laser is used to laser cut heart valveleaflets out of a sheet of equine pericardial tissue about 0.35-0.55 mmthick. The laser system preferably is an M-series laser engraving andcutting system available from Universal Laser Systems, Inc. This deviceemploys a 30-watt, pulsed, sealed CO₂ laser. The CO₂ laser produceslaser light with a characteristic wavelength of 10.6 μm. Mostnon-metals, including equine pericardial tissue, are highly absorptiveof laser energy at this wavelength, and also exhibit low thermalconductivity to such laser energy. Hence, the CO₂ laser is especiallyadvantageous for cutting pericardial tissue because the tissue absorbsand is vaporized by the CO₂ laser light but very little or no thermalenergy is conducted to regions of the tissue that are not being cut.Only the boundary/edge of the cut is melted, effectively forming a weld.

In the preferred embodiment, a sheet of equine pericardium is placed onthe support surface 80. An operator directs the computer 74 to actuatethe laser system 72, which cuts leaflets out of the sheet according tothe prescribed pattern 96. To help maintain the tissue in goodcondition, it preferably is kept moist when being cut.

When cutting equine pericardium, the laser preferably is operated at apower of about 7.5 watts (joules/second). The laser can cut at a linearspeed of about 1 inch per second, a pulse rate of about 1,000 pulses perinch (PPI), and a laser spot diameter of about 0.003 inches.

A measurement of laser energy per pulse is computed by using thefollowing equation (1):[laser energy per pulse(joules/pulse)]=[power(joules/second)]/([cuttingspeed(inches/second)]×[pulse rate(pulses/inch)]).

For the above embodiment, the laser energy per pulse is about:(7.5 joules/second)/((1 inch/second)×(1,000 pulses/inch))=0.0075joules/pulse.

Other materials, such as bovine or other kinds of pericardium tissuesand laminar materials can also be advantageously laser cut with a CO₂laser as discussed above. In another preferred embodiment wherein suchmaterials, including equine pericardium, are laser cut, about 0.005-0.5joules of laser energy are supplied per pulse, with a laser spot size ofabout 0.002 to 0.005 inches in diameter, a cutting speed of about 1inch/second, and a pulse rate of about 1,000 PPI. More preferably, about0.005-0.02 joules of laser energy are supplied per pulse. For theUniversal Laser Systems M-series laser discussed above, the followingsample settings enable laser cutting within the above-discussedparameters: a 1.5 Lens, 20% power setting, 3.4% speed, 1,000 PPI and1,000 dots per inch.

It is to be understood that if parameters such as the pulse rate andcutting speed are adjusted, corresponding adjustments to otherparameters can be made so that the energy imparted to the materialsubstantially stays within the desired parameters. In this manner, agenerally uniform weld can be formed along a cut edge withoutdiscoloring the edge or imparting excessive heat to other portions ofthe segment.

It is also to be understood that other types of lasers, such as anerbium laser that generates a laser beam having a wavelength of about2.7-3.0 μm, can suitably be used to cut segments. Such alternativelasers can be operated at settings so that the cut edges are welded asdiscussed above.

Alternative techniques may be employed for laser cutting of segments foruse in prosthetics, such as disclosed in U.S. Patent ApplicationPublication No. US 2002/0091441, which was published on Jul. 11, 2002.The entire disclosure of this publication is hereby incorporated hereinby reference.

Various types of tissue and man-made materials can be cut with a laserby using generally the same principles as discussed above. For example,other types of laminar tissue can be cut so that the cut edges arewelded and have a generally uniform consistency with little or nodiscoloration. Similarly, for man-made materials such as woven orknitted polymers, the cut edges preferably are melted so that fraying ofthe woven filaments or yarns is minimized or avoided, but discolorationis also avoided.

With reference next to FIG. 9, an embodiment of a laser cuttingapparatus for cutting curved or tubular materials is illustrated. Thisembodiment is substantially similar to the embodiment presented in FIG.6 except that the support surface 80 comprises a rotary axis 104configured to accept a tubular source material 106. In addition tovertical movement about a Z-axis, the rotary axis 104 is adapted torotate in order to help position the tubular source material 106 in anadvantageous cutting position relative to the focused laser beam 88.

Although this invention has been disclosed in the context of certainpreferred embodiments and examples, it will be understood by thoseskilled in the art that the present invention extends beyond thespecifically disclosed embodiments to other alternative embodimentsand/or uses of the invention and obvious modifications and equivalentsthereof. In addition, while a number of variations of the invention havebeen shown and described in detail, other modifications, which arewithin the scope of this invention, will be readily apparent to those ofskill in the art based upon this disclosure. It is also contemplatedthat various combinations or subcombinations of the specific featuresand aspects of the embodiments may be made and still fall within thescope of the invention. Accordingly, it should be understood thatvarious features and aspects of the disclosed embodiments can becombined with or substituted for one another in order to form varyingmodes of the disclosed invention. Thus, it is intended that the scope ofthe present invention herein disclosed should not be limited by theparticular disclosed embodiments described above, but should bedetermined only by a fair reading of the claims that follow.

1. A method of cutting tissue for use as an implantable medical prosthesis comprising: providing a sheet of tissue having at least two layers; operating a laser cutting apparatus at a power and pulse rate such that a laser beam directed on the sheet of, tissue fuses the at least two layers of the tissue together along a laser cut edge without significantly burning the tissue adjacent the laser cut edge; and forming a tissue segment from the sheet of tissue.
 2. The method of claim 1, wherein the tissue comprises pericardium.
 3. The method of claim 1, wherein the power and pulse rate of the laser cutting apparatus are selected so that there is substantially no discoloration of the tissue along the laser cut edge.
 4. The method of claim 1, wherein forming the tissue segment comprises forming a plurality of tissue segments; and wherein the method further comprises attaching the plurality of tissue segments to one another to form a prosthesis.
 5. The method of claim 1, wherein operating the laser cutting apparatus comprises operating the laser beam in a pulsed manner and supplying between about 0.005-0.5 joules of laser energy per pulse.
 6. The method claim 1, wherein operating the laser cutting apparatus comprises operating at a cutting speed of about 1 inch per second and a pulse rate of about 1000 pulses per inch.
 7. The method of claim 1, wherein operating the laser cutting apparatus comprises operating at a wavelength of about 10.6 microns.
 8. The method of claim 1, wherein operating the laser cutting apparatus comprises operating at a wavelength of about 2.7-3.0 microns.
 9. The method of claim 1, wherein operating the laser cutting apparatus comprises producing laser energy at about 0.0075 joules per pulse and a laser spot diameter of about 0.003 inches.
 10. The method of claim 1, wherein operating the laser cutting apparatus comprises producing laser energy between about 0.005 and 0.02 joules.
 11. The method of claim 1, wherein forming the tissue segment comprises forming the tissue segment for cardiac sot tissue repair and reconstruction.
 12. The method of claim 11, wherein forming the tissue segment comprises forming a plurality of tissue segments, wherein one tissue segment is used for soft tissue repair and reconstruction.
 13. The method of claim 1, wherein forming the tissue segment comprises forming several segments, and wherein the method further comprises constructing a tissue valve with the tissue segments.
 14. The method of claim 2, wherein the pericardium comprises bovine or equine pericardium.
 15. A method of producing an implantable medical tissue prosthesis comprising: providing a sheet of pericardium, wherein the pericardium includes at least two tissue layers; and cutting a segment of tissue out of the sheet of pericardium using a laser cutting apparatus configured to generate laser energy at a power and pulse rate such that said laser energy welds the layers of the pericardium together along a laser cut edge without significantly burning the pericardium adjacent the laser cut edge.
 16. The method of claim 15 further comprising forming a tissue implant from the pericardium tissue.
 17. The method of claim 16, wherein forming the tissue implant includes forming the tissue implant for tissue repair or tissue reconstruction.
 18. The method of claim 15, wherein the pericardium is bovine pericardium or equine pericardium. 